Quotes

One last issue that causes its share of confusion is quotes. In Linux, there are three kinds of quotes: double-quotes ("), single-quotes ('), and back-quotes(``) (also called back-ticks). On most US keyboards, the single-quotes and double-quotes are on the same key, with the double-quotes accessed by pressing Shift and the single-quote key. Usually this key is on the right-hand side of the keyboard, next to the Enter key. On a US-American keyboard the back-quote is usually in the upper left-hand corner of the keyboard, next to the 1.

To best understand the difference between the behavior of these quotes, I need to talk about them in reverse order. I will first describe the back-quotes, or back-ticks.

When enclosed inside back-ticks, the shell interprets something to mean "the output of the command inside the back-ticks." This is referred to as command substitution, as the output of the command inside the back-ticks is substituted for the command itself. This is often used to assign the output of a command to a variable. As an example, lets say we wanted to keep track of how many files are in a directory. From the command line, we could say

ls | wc

The wc command gives me a word count, along with the number of lines and number of characters. The | is a "pipe" symbol that is used to pass the output of one command through another. In this example, the output of the ls command is passed or piped through wc. Here, the command might come up as:

7 7 61

However, once the command is finished and the value has been output, we can only get it back again by rerunning the command. Instead, If we said:

count=`ls |wc`

The entire line of output would be saved in the variable count. If we then say echo $count, we get

7 7 61

showing me that count now contains the output of that line. If we wanted, we could even assign a multi-line output to this variable. We could use the ps command, like this

trash=`ps`

then we could type in

echo $trash

which gives us:

PID TTY TIME CMD 29519 pts/6 00:00:00 bash 12565 pts/6 00:00:00 ps

This is different from the output that ps would give when not assigned to the variable trash:

PID TTY TIME CMD
29519 pts/6 00:00:00 bash
12564 pts/6 00:00:00 ps


The next kind of quote, the single-quote ('), tells the system not to do any expansion at all. Lets take the example above, but this time, use single quotes:

count='ls |wc'

If we were to now type

echo $count

we would get

ls |wc

And what we got was exactly what we expected. The shell did no expansion and simply assigned the literal string "ls | wc" to the variable count. This even applies to the variable operator "$." For example, if we simply say

echo '$LOGNAME'

what comes out on the screen is

$LOGNAME

No expansion is done at all and even the "$" is left unchanged.

The last set of quotes is the double-quote. This has partially the same effect as single-quotes, but to a limited extent. If we include something inside of double-quotes, everything loses its special meaning except for the variable operator ($), the back-slash (\), the back-tick (`), and the double-quote itself. Everything else takes on its absolute meaning. For example, we could say

echo "`date`"

which gives us

Wed Feb 01 16:39:30 PST 1995

This is a round-about way of getting the date, but it is good for demonstration purposes. Plus, I often use this in shell scripts when I want to log something and keep track of the date. Remember that the back-tick first expands the command (by running it) and then the echo echoes it to the screen.

That pretty much wraps up the quote characters. For details on other characters that have special meaning to the shell check out the section on regular expressions. You can get more details from any number of references books on Linux or UNIX in general (if you need it). However, the best way to see what's happening is to try a few combinations and see if they behave as you expect.

Previously, I mentioned that some punctuation marks have special meaning, such as *, ?, and [ ]. In fact, most of the other punctuation marks have special meaning, as well. We'll get into more detail about them in the section on basic shell scripting.

It may happen that you forget to close the quotes, and you end up on a new line that starts with (typically) a greater than symbol >. This is the secondary prompt (PS2) and is simply telling you that your previous line continues. You can continue the line and the close the quotes later, like this:

VAR="Now is the time for all good admins
> to come to the aid of their operating system."

It is as if you wrote the entire line at once.

Sometimes it is necessary to include the literal quotes in your output variable. This is a problem because your shell interprets the quotes before assinging the value to the variable. To get around this you need to "escape" or "protect" the quotes using a backslash", like this:

echo \"hello, world\"